The present invention relates to a solid state imaging device including an imaging region which has a plurality of pixels over a semiconductor substrate, a camera, and a method for fabricating a solid state imaging device.
A MOS (Metal Oxide Semiconductor) solid state imaging device is an image sensor wherein a signal accumulated in a photodiode which constitutes a corresponding pixel is read by an amplification circuit which includes a MOS transistor. The MOS solid state imaging device is advantageously capable of low voltage operation and high-speed charge reading and can be integrated with peripheral circuits into one chip.
Thus, the MOS solid state imaging devices have been receiving attentions as to the uses in small cameras for personal computers and portable devices, such as mobile phones. In recent years, in the MOS solid state imaging devices, reduction in cell size and improvement in sensitivity have been especially demanded.
FIG. 8 is a circuit diagram showing an example of a circuit structure of a generally-employed MOS solid state imaging device. It should be noted that the circuit structure shown in FIG. 8 is applicable not only to a conventional solid state imaging device but also to a solid state imaging device of the present invention. Referring to FIG. 8, the generally-employed MOS solid state imaging device includes, on one substrate, an imaging region 27 in which a plurality of pixels 26 are arranged in a matrix, a vertical shift register 28 and horizontal shift register 29 for selecting the pixels, a timing generation circuit 30 for supplying necessary pulses to the vertical shift register 28 and horizontal shift register 29.
Each of the pixels 26 arranged over the imaging region 27 is formed by a photodiode region 21 for photoelectric conversion and a MOS transistor accompanying thereto. Electric charges accumulated in the photodiode region 21 are transferred by a transfer transistor 22 to a floating diffusion section 36. The drain of the floating diffusion section 36 also serves as the source of a reset transistor 23 which is connected to a power supply 33. The gate of an amplification transistor 24 is connected to the floating diffusion section 36, and the drain of the amplification transistor 24 is connected to the power supply 33. The source of the amplification transistor 24 is connected to the drain of a selection transistor 25. The source of the selection transistor 25 is connected to an output pulse line 35.
The gate of the transfer transistor 22, the gate of the reset transistor 23, and the gate of the selection transistor 25 are respectively connected to output pulse lines 31, 32, and 34 extending from the vertical shift register 28.
The MOS solid state imaging device includes a device isolation region for isolating the photodiode region 21 and corresponding MOS transistor formed on the semiconductor substrate. The device isolation region includes LOCOS (Local Oxidation of Silicon), which is a thermal oxidation film in general. However, when LOCOS is used, it is necessary to extend the width of the device isolation region in order to achieve a desired device separation characteristic. Further, since the formation of LOCOS causes a bird's beak, it is necessary to secure a sufficient width for an active region. Accordingly, it is necessary to increase the area of the device isolation region for one pixel and the area of the active region for one pixel. Thus, it is difficult to reduce the cell size.
A countermeasure against the above-described problem is a conventional technique disclosed in Japanese Laid-Open Patent Publication No. 2004-39832, which is described below. FIG. 9 is a cross-sectional view showing the structure of a photodiode region in a conventional solid state imaging device.
As shown in FIG. 9, the uppermost level of a silicon substrate 53 is a thin P+ silicon layer 56 for preventing leakage of electric charges from its surface. Provided under the P+ silicon layer 56 is a photodiode 62 which includes an N-type silicon layer (signal charge accumulation region) 54 and a P− silicon layer 55 provided under the N-type silicon layer 54.
A device isolation region 63 is provided around the photodiode 62 at the top (i.e., upper) surface of the silicon substrate 53. The device isolation region 63 extends from the top surface of the silicon substrate 53 to a depth substantially equal to the lower surface of the N-type silicon layer 54. The device isolation region 63 has a STI (Shallow Trench Isolation) structure. The device isolation region includes a silicon oxide (SiO2) film 61 which covers the inner surface of the well and an insulating film 52 of SiO2, or the like, which is provided over the silicon oxide film 61 to fill the well. With this structure, at the top surface of the silicon substrate 53, the photodiode 62 is electrically isolated from neighboring devices.
A portion of the silicon substrate 53 which is in contact with the bottom of the photodiode 62 is a P-type deep well 59. The P+ silicon layer 56 and the P-type deep well 59 are electrically connected through a P+ channel stopper layer 57 which covers the side and bottom surfaces of the device isolation region 63 and a P-type surface well 58 and P-type plug well 60 which are provided under the P+ channel stopper layer 57. With this structure, the N-type signal charge accumulation region (the N-type silicon layer 54) is also electrically isolated from the neighboring devices in the silicon substrate 53. Thus, in this structure, leakage of signal charges is small.
When light impinging on a light receiving region 51 of the photodiode 62 (a portion of the silicon substrate 53 which is surrounded by the device isolation region 63) reaches a PN junction of the N-type silicon layer 54 and the P+ silicon layer 56 or P− silicon layer 55, the light is converted to holes and electrons, and signal charges (electrons) which are determined according to the amount of the impinging light are accumulated in the N-type silicon layer (signal charge accumulation region) 54.
In this conventional example, the device isolation region 63 has a STI structure, and therefore, no bird's beak occurs, and the device isolation region does not extend into the light receiving region 51. Therefore, it is not necessary to secure a sufficient width for the active region of the MOS transistor. Accordingly, reduction in area of the light receiving region 51 is prevented, so that a large area can be secured for the light receiving region 51. In the device isolation region 63 of the STI structure, the width of the insulating material necessary for device isolation is narrow as compared with the LOCOS structure, and the like. Therefore, in the case of employing the STI structure as a device isolation, the area of the device isolation structure itself is reduced. Accordingly, the sensitivity of the photodiode is improved, and the size of one pixel is decreased.
However, in the above-described conventional example, a portion of the silicon substrate 53 which covers the side and bottom surfaces of the device isolation region 63 is a P+ channel stopper layer 57 which is formed by ion implantation. Therefore, in thermal processes for fabrication of the MOS solid state imaging device, the P+ channel stopper layer 57 extends so that the N-type silicon layer 54 in which photoelectrically-converted charges are to be accumulated is diminished. Especially in a solid state imaging device having a cell size of 3 μm or less, the dynamic range greatly decreases for the above causes.
When forming the device isolation region of the STI structure, an edge portion at the bottom of the device isolation region 63 locally has tensile stress in the silicon substrate 53. Around the bottom of the device isolation region 63 is the N-type silicon layer 54 in which charges are to be accumulated. In addition to the photoelectrically-converted charges, charges resulting from defects generated by the stress are also accumulated in the N-type silicon layer 54 (the signal charge accumulation region of the photodiode). That is, in addition to the charges generated when light impinges on the light receiving region, unnecessary charges are generated even when light does not impinge on the light receiving region, and the generated unnecessary charges are accumulated in the N-type silicon layer 54. These charges cause a variation of characteristics between pixels and a white blemish (white point) which occurs when there is no impinging light. As a result, the sensitivity of the photodiode deteriorates.